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Tear Jerker: A Canterlot Wedding Aftermath
The immense breakdown Twilight suffers throughout the story. It's pretty tragic seeing her become so psychologically broken due to the events of the wedding, but what makes it worse is the fact on that it was caused by her own best friends, her brother, and her own mentor.
Twilight's friends saying there goodbyes when she is given the choice to stay in Ponyville or start over in Canterlot.